The present invention relates to optical modulators, and more specifically to a technique for reducing noise and distortion in the output of an external optical intensity modulator.
Recently, there has been a growing interest in the development of analog, amplitude modulated optical communication systems. In comparison with digital systems, analog communication systems provide an efficient use of bandwidth. This is particularly useful in cable television (CATV) transmission system applications, where it is necessary to transmit a large number of video channels through an optical fiber. Compatibility with existing equipment is achieved by using the same signal format for optical transmission that is in use for coaxial cable signal transmission.
In order to transmit an information signal (e.g., a television signal) over an optical fiber, a light beam ("carrier") must be modulated with the information signal. The "electrooptic effect" has been advantageously used to provide modulators for this purpose. For example, electrooptic modulators using miniature guiding structures are known which operate with a low modulating power.
In electrooptic modulators, the electric field induced linear birefringence in an electrooptic material produces a change in the refractive index of the material which, in turn, impresses a phase modulation upon a light beam propagating through the material. The phase modulation is converted into intensity modulation by the addition of polarizers or optical circuitry. Ideally, an electrooptic modulator should have a linear relationship between its output optical power and the applied modulating voltage.
In a "Mach Zehnder" type electrooptic modulator, an optical carrier (laser beam) is split into two paths. At least one path is electrically phase modulated. The two signals are then recombined in an interferometer to provide an intensity modulated carrier. Typically, lithium niobate (LiNbO.sub.3) is used as the electrooptic material. Waveguides in such materials are readily formed by titanium indiffusion.
The output power curve of a Mach Zehnder modulator is nonlinear. Practical analog optical communications systems, however, demand a high linearity. See, for example, W. I. Way, "Subcarrier Multiplexed Lightwave System Design Considerations for Subscriber Loop Applications", J. Lightwave Technol., Vol 7, pp. 1806-1818 (1989). Modulator nonlinearities cause unacceptable harmonic and intermodulation distortions. When it is necessary to communicate a large number of channels, as in a CATV application, intermodulation distortions ("IMD") can impose serious limitations on the system performance. In principle, the second order IMD can be filtered out if the bandwidth is less than one octave. However, CATV transmission systems operate with bandwidths of many octaves. Third order IMD can only be eliminated by using devices with linear characteristics.
Another type of external optical modulator is the acoustooptic modulator. In these devices, the phase grating created by an acoustic wave through the photoelastic effect can either diffract a light beam into many orders as in the Raman-Nath regime of operation or deflect a light beam into a single order as in the Bragg regime. In either regime, intensity modulation of moderate bandwidth is easily accomplished without regard to the polarization of the incident light. At present, the bandwidth of acoustooptic modulators is limited to about a few hundred megahertz by practical considerations of the high frequency transducer design. Guidelines for the selection of acoustooptic materials for device applications are discussed in D. A. Pinnow, "Guidelines for the Selection of Acoustooptic Materials", IEEE J. Quantum Electron., Vol. QE-6, pp. 223-238, Apr. 1970. A review of acoustooptic materials and techniques for light deflection is presented by N. Uchida and N. Niizeki, "Acoustooptic Deflection Materials and Techniques", Proc. IEEE, Vol. 61, pp. 1073-1092, Aug. 1973. Acoustooptic modulators also exhibit a nonlinear relationship between output optical power and the applied modulating voltage. As a result, IMD must be reduced to provide practical operation in applications such as cable television transmission.
Typical CATV fiber optic systems using frequency division multiplexed amplitude modulated (AM-FDM) signals will modulate the light output of a laser diode proportionally to the composite AM signal of the cable television FDM spectrum. Lasers with adequate power output and low distortion are expensive and difficult to make. An alternate scheme is to use a high power laser and externally modulate the laser beam. As noted above, known external modulators are nonlinear, although a small linear range of operation is generally available. In order to operate such modulators with low distortion, a high optical carrier input power and small depth of modulation must be used over the limited linear range. When a high power optical signal is output for transmission, the receiving diode yields a large shot noise product. This, coupled with the low modulation percentage, gives a lower than desirable signal to noise ratio in the receiver.
It would be advantageous to provide an external optical modulator that reduces the nonlinear distortion, and particularly second order distortion, of the modulated signal. It would be further advantageous to provide such a modulator that outputs a reduced optical carrier power, to increase the effective modulation of individual carriers and reduce the receiver shot noise. The present invention provides an external optical modulator having the aforementioned advantages.